Circle of latitude

A circle of latitude on Earth is an abstract eastwest circle connecting all locations around Earth (ignoring elevation) at a given latitude.

Circles of latitude are often called parallels because they are parallel to each other; that is, any two circles are always the same distance apart. A location's position along a circle of latitude is given by its longitude. Circles of latitude are unlike circles of longitude, which are all great circles with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from the Equator increases. Their length can be calculated by a common sine or cosine function. The 60th parallel north or south is half as long as the Equator (disregarding Earth's minor flattening by 0.3%). A circle of latitude is perpendicular to all meridians.

The latitude of the circle is approximately the angle between the Equator and the circle, with the angle's vertex at Earth's centre. The equator is at 0°, and the North Pole and South Pole are at 90° north and 90° south, respectively. The Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle.

There are 89 integral (whole degree) circles of latitude between the equator and the Poles in each hemisphere, but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree (e.g. 34.637°N) or with minutes and seconds (e.g. 22°14'26"S). There is no limit to how precisely latitude can be measured, and so there are an infinite number of circles of latitude on Earth.

On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which projection is used to map the surface of the Earth onto a plane. On an equirectangular projection, centered on the equator, the circles of latitude are horizontal, parallel, and equally spaced. On other cylindrical and pseudocylindrical projections, the circles of latitude are horizontal and parallel, but may be spaced unevenly to give the map useful characteristics. For instance, on a Mercator projection the circles of latitude are more widely spaced near the poles to preserve local scales and shapes, while on a Gall–Peters projection the circles of latitude are spaced more closely near the poles so that comparisons of area will be accurate. On most non-cylindrical and non-pseudocylindrical projections, the circles of latitude are neither straight nor parallel.

Arcs of circles of latitude are sometimes used as boundaries between countries or regions where distinctive natural borders are lacking (such as in deserts), or when an artificial border is drawn as a "line on a map", which was made in massive scale during the 1884 Berlin Conference, regarding huge parts of the African continent. North American nations and states have also mostly been created by straight lines, which are often parts of circles of latitudes. For instance, the northern border of Colorado is at 41°N while the southern border is at 37°N. Roughly half the length of border between the United States and Canada follows 49°N.

Tissot indicatrix world map Mercator proj
The Mercator projection of a world map. The angles are untrue for area, especially at high latitudes. Also note increasing distances between the latitudes towards the poles and the parallel lines of longitude. The only true world map is the globe. The Mercator projection comes from a globe inside a cylinder.
Usgs map mercator
The Mercator projection and its use on a world map. This projection first came into use in the 16th century by the Dutch.

Major circles of latitude

World map with major latitude circles
Diagram showing the locations of the five major circles of latitude on an equirectangular projection of the Earth.
Axial tilt vs tropical and polar circles
Relationship between Earth's axial tilt (ε) to the tropical and polar circles

There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed (90 degrees from Earth's axis of rotation) but the latitudes of the other circles depend on the tilt of this axis relative to the plane of Earth's orbit, and so are not perfectly fixed. The values below are for 6 April 2019:[1]

These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones.

Equator

The equator is the circle that is equidistant from the North Pole and South Pole. It divides the Earth into the Northern Hemisphere and the Southern Hemisphere. Of the parallels or circles of latitude, it is the longest, and the only 'great circle' (a circle on the surface of the Earth, centered on Earth's center). All the other parallels are smaller and centered only on Earth's axis.

World map with equator
Equator

Polar Circles

The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the June and December solstices respectively). Similarly, the Antarctic Circle marks the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the December and June Solstices respectively).

The latitude of the polar circles is equal to 90° less the Earth's axial tilt.

World map with arctic circle
Arctic Circle
 
World map with antarctic circle
Antarctic Circle

Tropical Circles

The Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the sun may be seen directly overhead (at the June solstice and December solstice respectively).

The latitude of the tropical circles is equal to the Earth's axial tilt.

World map with tropic of cancer
Tropic of Cancer
 
World map with tropic of capricorn
Tropic of Capricorn

Movement of the Tropical and Polar Circles

By definition, the positions of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle all depend on the tilt of the Earth's axis relative to the plane of its orbit around the sun (the "obliquity of the ecliptic"). If the Earth were "upright" (its axis at right angles to the orbital plane) there would be no Arctic, Antarctic, or Tropical circles: at the poles the sun would always circle along the horizon, and at the equator the sun would always rise due east, pass directly overhead, and set due west.

The positions of the Tropical and Polar Circles are not fixed because the axial tilt changes slowly – a complex motion determined by the superimposition of many different cycles (some of which are described below) with short to very long periods. In 2000 the mean value of the tilt was about 23° 26′ 21″.

The main long-term cycle causes the axial tilt to fluctuate between about 22.1° and 24.5° with a period of 41,000 years. Currently, the average value of the tilt is decreasing by about 0.47″ per year. As a result, (approximately, and on average) the Tropical Circles are drifting towards the equator (and the Polar Circles towards the poles) by 15 metres per year, and the area of the Tropics is decreasing by 1,100 square kilometres (420 sq mi) per year.

The Earth's axial tilt has additional shorter-term variations due to nutation, of which the main term, with a period of 18.6 years, has an amplitude of 9.2" (corresponding to almost 300 metres north and south).[2] There are many smaller terms, resulting in varying daily shifts of some metres in any direction.

Finally, the Earth's rotational axis is not exactly fixed in the Earth, but undergoes small fluctuations (on the order of 15 meters) called polar motion, which have a small effect on the Tropics and Polar Circles and also on the Equator.

Short-term fluctuations over a matter of days do not directly affect the location of the extreme latitudes at which the sun may appear directly overhead, or at which 24-hour day or night is possible, except when they actually occur at the time of the solstices. Rather, they cause a theoretical shifting of the parallels, that would occur if the given axis tilt were maintained throughout the year.

Other planets

These circles of latitude can be defined on other planets with axial inclinations relative to their orbital planes. Objects such as Pluto with tilt angles greater than 45 degrees will have the tropic circles closer to the poles and the polar circles closer to the equator.

Other notable parallels

A number of sub-national and international borders were intended to be defined by, or are approximated by, parallels. Parallels make convenient borders in the northern hemisphere because astronomic latitude can be roughly measured (to within a few tens of meters) by sighting the North Star.

Missouri Compromise Line

Elevation

Circle of latitude elevation
Note that the features of the ellipsoid cross-section (orange) in this image are exaggerated with respect to those of the Earth.

Elevation has an effect on a location with respect to the plane formed by a circle of latitude. Since (in the geodetic system) altitude and depth are determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations (i.e., lying along this normal)—no longer lie within this plane. Rather, all points sharing the same latitude—but of varying elevation and longitude—occupy the surface of a truncated cone formed by the rotation of this normal around the Earth's axis of rotation.

See also

References

  1. ^ "''Trópico en movimiento'' (in Spanish)". Groups.google.com. Retrieved 2014-05-13.
  2. ^ "Basics of Space Flight, Chapter 2". Jet Propulsion Laboratory. Jet Propulsion Laboratory/NASA. 2013-10-29. Retrieved 2015-03-26.
  3. ^ "Maritime Delimitation between Nicaragua and Honduras in the Caribbean Sea (Nicaragua v. Honduras)" (PDF). Archived from the original (PDF) on 2015-09-24. Retrieved 2014-05-13.

External links

15th parallel south

The 15th parallel south is a circle of latitude that is 15 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.

20th parallel south

The 20th parallel south is a circle of latitude that is 20 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.

25th parallel south

The 25th parallel south is a circle of latitude that is 25 degrees south of the Earth's equatorial plane, just south of the Tropic of Capricorn. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.

30th parallel south

The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.

At this latitude the sun is visible for 14 hours, 5 minutes during the December solstice and 10 hours, 13 minutes during the June solstice. On December 21, the sun is at 83.83 degrees up in the sky and at 36.17 degrees on June 21.

35th parallel north

The 35th parallel north is a circle of latitude that is 35 degrees north of the Earth's equatorial plane. It crosses Africa, the Mediterranean Sea, Asia, the Pacific Ocean, North America and the Atlantic Ocean.

In the United States, the parallel defines the southern border of Tennessee, and the border between North Carolina and Georgia, as well as the southernmost point of Nevada.

At this northern latitude, the Sun is visible for 14 hours, 31 minutes on its summer solstice (in June) and for 9 hours, 48 minutes on its winter solstice (in December).

35th parallel south

The 35th parallel south is a circle of latitude that is 35 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, the Indian Ocean, Australasia, the Pacific Ocean and South America.

At this latitude the sun is visible for 14 hours, 31 minutes during the December solstice and 9 hours, 48 minutes during the June solstice.

40th parallel south

The 40th parallel south is a circle of latitude that is 40 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, the Indian Ocean, Australasia, the Pacific Ocean and South America. Its long oceanic stretches are the northern domain of the Roaring Forties.

On 21 June 2018, the sun is at 26.17° in the sky and at 73.83° on 21 December, in King Island, Tasmania, which is near the 40th parallel.

45th parallel south

The 45th parallel south is a circle of latitude that is 45° south of the Earth's equator.

It is the line that marks the theoretical halfway point between the equator and the South Pole. The true halfway point is 16.2 km (10.1 mi) south of this parallel because Earth is not a perfect sphere, but bulges at the equator and is flattened at the poles.Unlike its northern counterpart, almost all (97%) of it passes through open ocean. It crosses the South Atlantic Ocean, the Indian Ocean, Australasia (New Zealand and just south of Tasmania), the Southern Ocean, and Patagonia.

At this latitude, daytime lasts for 15 hours, 37 minutes during the December solstice and 8 hours, 46 minutes during the June solstice.

50th parallel south

The 50th parallel south is a circle of latitude that is 50 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, the Indian Ocean, the Pacific Ocean and South America.

At this latitude the sun is visible for 16 hours, 22 minutes during the December solstice and 8 hours, 4 minutes during the June solstice. On December 21, the sun is at 63.83 degrees in the sky and on June 21, the sun is at 16.17 degrees in the sky.

55th parallel south

The 55th parallel south is a circle of latitude that is 55 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, the Indian Ocean, the Pacific Ocean and South America.

At this latitude the sun is visible for 17 hours, 22 minutes during the December solstice and 7 hours, 10 minutes during the June solstice.

5th parallel south

The 5th parallel south is a circle of latitude that is 5 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, Australasia, the Pacific Ocean and South America.

60th parallel south

The 60th parallel south is a circle of latitude that is 60 degrees south of the Earth's equatorial plane. No land lies on the parallel — it crosses nothing but ocean. The closest land is a group of rocks north of Coronation Island (Melson Rocks or Governor Islands) of the South Orkney Islands, which are about 54 km south of the parallel, and Thule Island and Cook Island of the South Sandwich Islands, which both are about 57 km north of the parallel (Thule island slightly closer).

The parallel marks the northern limit of the Southern Ocean (though some organisations and countries, notably Australia, have other definitions) and of the Antarctic Treaty System. It also marks the southern boundary of the South Pacific Nuclear-Weapon-Free Zone and the Latin American Nuclear-Weapon-Free Zone.

At this latitude the sun is visible for 18 hours, 52 minutes during the summer solstice and 5 hours, 52 minutes during the winter solstice. On December 21, the sun is at 53.83 degrees up in the sky and 6.17 degrees on June 21.

The latitudes south of this parallel are often referred to as the Screaming 60s due to the prevailing high-speed, westerly winds which can generate large waves in excess of 15 m (50 ft) and peak wind speeds over 145 km/h (90 mph).

65th parallel north

The 65th parallel north is a circle of latitude that is 65 degrees north of the Earth's equatorial plane. It crosses the Atlantic Ocean, Europe, Asia and North America.

At this latitude the sun is visible for 22 hours, 2 minutes during the summer solstice and 3 hours, 35 minutes during the winter solstice.

65th parallel south

The 65th parallel south is a circle of latitude that is 65 degrees south of the Earth's equatorial plane. It crosses the Southern Ocean and Antarctica.

At this latitude the sun is visible for 22 hours, 02 minutes during the December solstice and 3 hours, 35 minutes during the June solstice.

70th parallel south

The 70th parallel south is a circle of latitude that is 70 degrees south of the Earth's equatorial plane in the Antarctic. The parallel passes through the Southern Ocean and Antarctica.

75th parallel north

The 75th parallel north is a circle of latitude that is 75 degrees north of the Earth's equatorial plane, in the Arctic. It crosses the Atlantic Ocean, Europe, Asia, the Arctic Ocean and North America.

75th parallel south

The 75th parallel south is a circle of latitude that is 75 degrees south of the Earth's equatorial plane in the Antarctic. The parallel passes through the Southern Ocean and Antarctica.

80th parallel north

The 80th parallel north is a circle of latitude that is 80 degrees north of the Earth's equatorial plane, in the Arctic. It crosses the Atlantic Ocean, Europe, Asia, the Arctic Ocean and North America.

80th parallel south

The 80th parallel south is a circle of latitude that is 80 degrees south of the Earth's equatorial plane.

It passes through Antarctica and Antarctic ice shelves.

Circles of latitude / meridians
Astronomers
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